Glandular epithelia, including the mammary and prostate glands, are composed of basal cells (BCs) and luminal cells (LCs) 1,2. Many glandular epithelia develop from multipotent basal stem cells (BSCs) that are replaced in adult life by distinct pools of unipotent stem cells 1,3-8. However, adult unipotent BSCs can reactivate multipotency under regenerative conditions and upon oncogene expression 3,9-13. This suggests that an active mechanism restricts BSC multipotency under normal physiological conditions, although the nature of this mechanism is unknown. Here we show that the ablation of LCs reactivates the multipotency of BSCs from multiple epithelia both in vivo in mice and in vitro in organoids. Bulk and single-cell RNA sequencing revealed that, after LC ablation, BSCs activate a hybrid basal and luminal cell differentiation program before giving rise to LCs-reminiscent of the genetic program that regulates multipotency during embryonic development 7. By predicting ligand-receptor pairs from single-cell data 14 , we find that TNF-which is secreted by LCs-restricts BC multipotency under normal physiological conditions. By contrast, the Notch, Wnt and EGFR pathways were activated in BSCs and their progeny after LC ablation; blocking these pathways, or stimulating the TNF pathway, inhibited regeneration-induced BC multipotency. Our study demonstrates that heterotypic communication between LCs and BCs is essential to maintain lineage fidelity in glandular epithelial stem cells. Genetic lineage tracing of BCs of the mammary gland (MG) under physiological conditions demonstrates that BCs are multipotent during embryonic development, giving rise to BCs and LCs that become lineage-restricted at the end of embryonic development and sustain only the BC lineage postnatally 1,3,7,8. Transplantation of adult unipotent MG BCs alone results in MG outgrowth composed of BCs and LCs 3,9-11 , demonstrating the multipotent potential of BCs when they are transplanted without LCs, and illustrating the discrepancy between the fate of BCs under physiological conditions and their potential under regenerative conditions. By contrast, when BCs are transplanted together with LCs, BCs maintain their unipotent basal fate 3 , suggesting that LCs might signal to BCs to restrict their multi-lineage differentiation.